High rate solar still and process

ABSTRACT

A method and apparatus for purifying raw feedwater. The raw feedwater is directed into an evaporator module submerged in a solar pond or other body of heated liquid. The evaporator module includes a rotating housing through which a plurality of spaced apart substantially horizontal open ended heat transfer tubes extend. A heating liquid is directed through the heat transfer tubes. The raw feedwater is distributed within the evaporator module so as to cause the feedwater to descend into heat transferring contact with the heat transfer tubes and thereby vaporize a portion of the feedwater. The vapor formed in the evaporator module is withdrawn from the evaporator module and directed to a suitable condensor. 
     A preferred embodiment of the evaporator module is disclosed which includes cavitation fins or urging the heated liquid through the evaporator module.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to solar stills and more specifically to the use of a specialized high rate falling film heat exchanger/evaporator submerged within a solar energy collection pond which utilizes the salt gradient principle. The evaporator component of the invention also has applicability submerged within other liquid bodies which contain sufficient heat for the desired use. The invention has particular application in geographical areas of high insolation, inadequate water supplies and relatively inexpensive land. The high rate solar still of the present invention may be used to purify sea water, brackish water, fresh water containing unusually high fouling impurities, and for the recycling of irrigation water; especially in those cases where alternative water treatment processes, including membranes, require prohibitively expensive pretreatment techniques in order to operate.

2. Description of the Prior Art

The art of producing drinking water from salt water by means of solar distillation is well known. Most designs of solar stills consist of floating or semi-submerged apparatus enclosed by material transparent to solar radiation wherein a pool of salinous water is allowed to heat up and vaporize. Condensation of the vapor is frequently accomplished by routing the vapor through a deeper level of seawater. Attempts have been made to improve the efficiency of solar energy collection by using lens and solar tracking devices. Examples of prior art are disclosed in U.S. Pat. Nos. 4,325,788, 4,276,122, 4,219,387, 4,172,767, 3,986,936, 3,703,443, 3,408,260, 3,357,898 and 4,151,046.

Systems have heretofore been constructed wherein flat plate solar collectors feed hot water to conventional multistage flash distillation plants. A demonstration scale system of such a system has been in operation in Mexico for several years. Another approach has been to use photovoltaic cells to generate electricity that operates a membrane separation desalination process.

Most of the prior art dealing with solar energy collection ponds, consists of articles published in various trade and scientific journals and, in particular, recent research conducted and published on salt gradient solar ponds by the U.S. Departments of Interior, Energy and Agriculture. In U.S. Pat. No. 4,110,172 an enclosed solar collection pond is disclosed containing salinous water with a depth of one to ten inches. This invention is focused on the rapid heating of the shallow pool of water and the recirculation of the heated air in order to raise the efficiency of the process.

A ditch similar in configuraton to the solar pond utilized in the present invention is described in U.S. Pat. No. 4,141,798. However, the solar still arrangement as described therein is substantially similar in principle to the above mentioned prior art which uses a flexible transparent plastic enclosure operating on a relatively shallow pool of salinous water, i.e., the greenhouse effect.

Applicant is not aware of any prior art which uses a salt gradient solar pond, with its superior temperature elevation capability and heat storage capability in the absence of insolation and/or in the cold season, in combination with a submerged evaporator device specially designed to take advantage of this stored heat in a space much smaller than required by shallow, non-salt gradient ponds, and constructed in such a way as to minimize the consequences of the corrosive environment.

The horizontal tube falling film evaporator has been in commercial desalination service since the late 1960's, in the single effect vapor compression mode, and more recently, in the multiple effect configuration using generated steam as an energy source. This evaporator design has also been used in connection with solar ponds and waste heat with the intent to produce power. However, in each such case the evaporator consisted of a conventional shell and tube arrangement situated external to the heat laden liquid body. In the recent past, solar ponds have been used to heat water flowing through submerged tubing for space heating purposes without severe corrosion problems.

A rotating disk distillation device is described in U.S. Pat. No. 3,764,483 which operates on the principle of a hydrodynamically applied thin film relying on the wiping action of a flexible blade. A multiple effect version of this device has been proposed for use with a salt gradient solar pond. However, the device was proposed to be situated external to the pond and is not suitable for submerged operation, nor is the evaporation principle the same as the present invention, i.e., wiped film disk versus horizontal tube falling film.

Accordingly, there appears to be no disclosure in the prior art of an evaporator submerged within the heat source, with exposed tube sheets, so as to reduce construction costs, reduce ambient heat loss, eliminate pumping of heating medium, and that rotates to (1) maximize the falling film heat transfer effect, (2) repeatedly expose the concentrate to heat transfer surface, (3) periodically submerge a large proportion of the heat transfer tubes to reduce scaling and (4) induce flowthrough of the heating medium, and that is constructed with a minimum use of welds to reduce corrosion. This combination of features cooperate to maximize the efficiency of the distillation process while minimizing space requirements compared to the prior art.

SUMMARY OF THE INVENTION

The present invention combines the unique heat collection and storage capability of the salt gradient solar pond and the superior efficiency of the falling film evaporator in such a way as to minimize construction cost, land requirements, and heat loss to the atmosphere.

Compared to other solar energy collection devices, the salt gradient solar pond has the unique capability of maintaining a useful quality and quantity of heat over a sufficient period of time to compensate for diurnal, week long, and even seasonal reductions in insolation. The typical depth of 6 to 10 feet also allows for the collection and storage of solar energy in a smaller space than is the case for other types of solar ponds. Salt gradient solar ponds, in actual practice, have been shown to generate a temperature up to 180° F. during the summer months.

The distillation process (evaporation) has the unique characteristic that the cost of construction and operation, as well as the product water quality, vary insignificantly with the degree of contamination of the feedwater. Hence, the lower the quality of the feedwater, the more economical is the distillation process as compared to other processes capable of removing dissolved inorganics, such as reverse osmosis membranes. Heretofore, the principle drawback to distillation has been the cost of energy to operate the process. The use of solar energy or waste heat, as prescribed herein, avoids this problem.

The high rate solar still of the present invention comprises an evaporator apparatus submerged within a salt gradient solar pond. The pond is preferably long and narrow in configuration in order to reduce the unsupported span and the construction cost of the evaporator support bridge and to allow the use of a factory fabricated impermeable pond liner. This configuration also reduces the risk of wind disturbance since the prevailing wind is usually on the north-south axis and perpendicular to the east-west orientation of the solar pond that maximizes insolation. The depth of the pond is dependent upon the maximum heat storage capability of the pond and the need to provide space for the proper operation of the evaporator. Depending on local conditions, the pond may lie partially below grade or it may be constructed entirely above grade within earthen dikes, or fabricated walls, to facilitate access to piping. In general, the sides of the pond will be sloped at such an angle that entirely earthen construction may be possible without producing shadows during periods of significant insolation.

The evaporator apparatus is submerged within the heat storage section of the salt gradient solar pond. Depending on the pure water production capacity desired, one or more evaporator modules may be placed along the full length of the pond. The evaporator module of the present invention is a form of cylindrical, horizontal tube falling film evaporator in which the tubes, on both ends, allow direct entry and passage therethrough of a salt solution heating medium. The raw feedwater enters the evaporator module through a supporting center shaft, falls through perforations and then from tube to tube, being repeatedly exposed to the heat from heating medium within the tubes.

The entire evaporator module rotates about its horizontal axis at a rate slow enough so as not to disturb the salt gradient but rapidly enough to ensure contact of the falling droplets with many of the tubes within the module. The rotation of the module also serves the purpose of agitating the increasingly concentrated liquid at the bottom of the evaporator module body and forming thin films of this liquid as the tubes emerge in their rotation. Flowthrough of the heating medium is induced through the rotation of the apparatus acting in connection with strategically oriented cavitation fins.

The vapor generated by the evaporation process exits at the opposite end of the center shaft and rises through an exit pipe and is directed to a condenser. Thus, the present invention has the effect of converting solar energy into pumping energy. Depending on the location of the point of use and the available head of the feedwater source, this invention may eliminate the need in the process for pumps constructed of costly metal alloys. In contrast, in conventional arrangements, wherein the evaporator is external to the solar pond, it is necessary to pump the saturated salt solution to the evaporator.

Periodically the evaporator must be flushed to remove concentrate. This may be accomplished by directing a large flow of raw feedwater through the evaporator module. The resultant mixture of raw feedwater and concentrate exit the module either up through the exit pipe or down though a drain, if provided. The evaporator module may be brought to the surface of the pond for inspection and maintenance by effecting changes in the bouyancy of the evaporator body in a manner similar to the way submarines remove or add ballast to rise or fall.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is an elevational view, partially in section, of a preferred embodiment of the invention, schematically depicting the raw water impoundment and the condenser.

FIG. 2 is a top plan view taken along line 2--2 in FIG. 1.

FIG. 3 is an elevational sectional view taken along line 3--3 in FIG. 2.

FIG. 4 is a sectional elevational view of an evaporator module constructed in accordance with the present invention.

FIG. 5 is an end view at the effluent end of a portion of the evaporator module as shown in FIG. 4.

FIG. 6 is a sectional view of the heat transfer tube and cavitation fin, taken along line 6--6 in FIG. 4.

FIG. 7 is a sectional view taken along line 7--7 in FIG. 6.

FIG. 8 is an elevation view of a preferred arrangement for raising and lowering the evaporator module.

DESCRIPTION OF PREFERRED EMBODIMENTS

Referring to FIGS. 1-3, a preferred embodiment of the high rate solar still system in accordance with the present invention is indicated by the reference numeral 10. Solar still system 10 comprises a salt gradient solar pond 12, having a plurality of evaporator modules 14 submerged therein. The solar pond is preferably of a long and narrow configuration to reduce the unsupported span and the construction cost of the bridge structure which supports the evaporator modules. The pond is preferably formed on the bottom by soil and on its sides by earthen dikes 16 and is overlayed with a suitable factory fabricated impermeable pond liner 18. Solar pond 12 is preferably dimensioned as follows: 8 feet to 20 feet in width; 4 feet to 12 feet in depth and may be of unlimited length.

The evaporator modules 14 are supported from support bridges 20, of suitable construction, which span the width of the solar pond 12, such that the evaporator modules are submerged in the heat storage section 22 of the solar pond 12 and oriented parallel to the length thereof. Depending on the pure water production capacity of the system, evaporator modules may be positioned along the full length of solar pond 12. Further, two or more transversely spaced modules may be supported from each bridge structure 20. Alternatively, the evaporator modules may be oriented perpendicular to the length of solar pond 12.

Referring to FIGS. 4 and 5, an evaporator module 14, constructed in accordance with the present invention, is depicted comprising an elongated cylindrical housing 24, sealed off by an influent end plate 26 and an effluent end plate 28. End plates 26 and 28 have a plurality of aligned openings 30 formed therein for receipt of open ended heat transfer tubes 32 therethrough. The respective end portions of heat transfer tubes 32 are suitably sealed to the end plates 26 and 28. A horizontally disposed hollow support shaft 34 extends through horizontally aligned openings 36 in plates 26 and 28 along the horizontal axis of housing 24. Shaft 34 is rigidly secured and sealed to plates 26 and 28 such that rotation thereof effects rotation of the evaporator module 14. The respective ends of shaft 34 are supported for rotation in a suitable manner as by a Teflon bearing and lip seal arrangement 38. The influent end 40 of shaft 34 is connected to a raw feedwater conduit 42 through a suitable fitting 44. Conduit 42 is in communication with a reservoir or source of raw feedwater indicated by the reference numeral 46 in FIG. 1. A control valve 48 is provided to control the flow of raw feedwater through conduit 42.

The entire evaporator module 14 rotates about its horizontal axis and is suitably driven in a conventional manner. For example, a chain and sprocket arrangement 50 secured to shaft 34 may be utilized in cooperation with a motor 52 supported on bridge 20. Alternatively, a hydraulic turbine (not shown) may be utilized to rotate shaft 34 and thereby rotate module 14.

Referring to FIG. 4, shaft 34 is provided with a plurality of openings 54 spaced along substantially the entire length within housing 24. The effluent end portion 56 of shaft 34 is closed off by a wall 58, positioned a short distance inward of end plate 28. Accordingly, all of the raw feedwater directed into shaft 34 through conduit 42 falls through openings 54 into the housing 24. The openings 54 are preferably sized and located in such a manner as to cause the raw feedwater to be distributed along the full length of shaft 34 in approximately uniform flow as the evaporator module 14 rotates about its horizontal axis.

Referring to FIGS. 4-7, heat transfer tubes 32 have influent end sections 60 and effluent end sections 62, which respectively extend outwardly of plates 26 and 28. In order to gently induce the passage of the salt solution medium from solar pond 12 into the influent end portions of tubes 32, the effluent end portions of tubes 32 are provided with cavitation fins 64. Cavitation fins 64 are preferably formed by excising a partial section from the effluent end sections 62, as shown in FIGS. 6 and 7, or, alternatively, by securing a flexible tube sleeve of the same shape. Each cavitation fin 64 is preferably oriented such that an imaginary radial line passing through the apex thereof is always tangent to an imaginary circle that defines the direction of rotation of the corresponding heat transfer tube 32.

The evaporator module 14 is rotated at a rate slow enough so as not to disturb the salt gradient but rapidly enough to ensure contact of the falling droplets passing through openings 54 with many of the tubes 32 within the evaporator module. The rotation also serves the purpose of agitating the increasingly concentrated liquid at the bottom of the evaporator housing 24 and forming thin films of this liquid as the tubes emerge in their rotation. This periodic submergence of some of the tubes also has the effect of reducing fouling and scaling. The optimum level of liquid in housing 24 is suitably controlled to maximize the evaporation process.

The vapor generated during the evaporation process exits the evaporator module 14 through openings 66 in the effluent end portion 56 of shaft 34 downstream from wall 58. The vapor rises up a vertical effluent conduit 68, secured to shaft 34 by a bearing and lip seal arrangement 38 and a fitting 44, and is directed to a heat exchanger/condenser 70, which condenses the vapor into pure product water in a conventional manner. This heat exchanger/condenser 70 may be either air cooled or watercooled. The condensed product water is then directed to its intended use or into a storage tank. The overall efficiency of the process may be improved by using the vapor to heat the incoming raw feedwater.

The evaporator module 14 is preferably constructed of corrosion resistant materials. The module is preferably assembled with gaskets, grommets and fasteners in such a way that it can be disassembled in the field by unskilled labor for the repair and replacement of components and for the removal of scale and foulants.

Periodic flushing of the evaporator module 14 to remove concentrate and to minimize scaling may be accomplished by allowing a large flow of feedwater to enter the evaporator module either from a higher elevation raw water storage impoundment or by pumping. The resultant mixture of raw water and concentrate exits through the openings 66 and is directed up the vertical exit effluent conduit 68, or alternately down a drain conduit 72, as shown in phantom lines in FIG. 4. The flushing operation is controlled by valve 48 and an effluent valve 74 to direct the concentrate to a disposal conduit 76, to prevent the flooding of the heat exchanger/condenser 70 and to prevent untimely drainage. The flushing function may also be accomplished by using the evaporator module in the manner of a compressed air ejector in order to reduce the quantity of wastewater.

Referring to FIG. 8, in accordance with a preferred embodiment of the invention, the evaporator module 14 and interconnected piping may be raised to the surface of the solar pond 12 for inspection and subsequent removal if necessary. First the feedwater valve 48 is closed and continued evaporation will increase the bouyancy of the module. Second, the upper ends of conduits 42 and 68 are released at quick disconnect fittings 77. The submerged module 14 and conduits 42 and 68 are allowed to rise, guided by pipe sleeves 78 secured to the bridge 20. To return the apparatus to its submerged operating position, water is inserted through the open feedwater piping to fill the evaporator body to the extent that it will sink in a manner similar to adding ballast to a ship or submarine. When a drain 72 is provided, disconnection is achieved with a quick disconnect coupling 80 and an extended operator rod 82. In this case the small amount of heating medium filling the open drain pipe can be drained into a container and returned to the pond. Alternatively, to facilitate removal of the evaporator from the pond surface the bridge may be raised by virtue of a pin and hinge arrangement (not shown) on one side of the bridge 20.

The evaporator module 14 described herein may also be submerged with a similar effect in any body of heated liquid. For example, a similar apparatus may be submerged within an electric power plant cooling pond. It would have the effect of simultaneously utilizing waste heat for production of pure water and providing cooling in lieu of a larger cooling pond or a conventional cooling tower. A similar apparatus may also be used as a wastewater concentrator. For example, cooling system or boiler blowdown water may be concentrated for economical disposal while at the same time producing purified water to be recycled. In this case it would be necessary to strengthen the module in such a way as to allow its use as a compressed air ejector during the flushing cycle to avoid dilution of the wastewater. 

What is claimed is:
 1. A solar still apparatus, comprising:(a) an elongated salt gradient solar pond defining a heat storage section containing a hot liquid; (b) an evaporator module supported within said solar pond for rotation about a substantially horizontal axis and submerged within said heat storage section thereof, comprising:(i) an elongated cylindrical housing sealed off by an influent end plate and an effluent end plate; (ii) a plurality of spaced apart substantially horizontal open ended heat transfer tubes positioned within said housing having influent and effluent end sections thereof respectively extending through said influent end plate and said effluent end plate for passage of hot liquid from said solar pond therethrough; (iii) a substantially hollow distribution shaft positioned within said housing having end sections thereof respectively extending through said influent end plate and said effluent end plate; (iv) said distribution shaft having a plurality of spaced apart openings formed therein; and (v) said effluent end section of said distribution shaft being closed off by a wall; (c) influent conduit means for directing feedwater into said distribution shaft for distribution of same through said openings in said shaft into said housing; (d) means for rotation of said evaporator module about a substantially horizontal axis so as to cause the feedwater to fall through said housing into heat transferring contact with said heat transfer tubes and thereby vaporize a portion of the feedwater; (e) means for withdrawing the vapor formed in said evaporator module; and (f) means for condensing the vapor withdrawn from said evaporator module.
 2. The invention as defined in claim 1 wherein said effluent end sections of said heat transfer tubes have cavitation fin means associated therewith which extend into said heat storage section for inducing the flow of hot liquid from said heat storage section into said influent end sections of said heat transfer tubes.
 3. The invention as defined in claim 2 wherein said cavitation fin means are formed by excising a partial section of the effluent end sections of said heat transfer tubes.
 4. The invention as defined in claim 2 wherein said heat transfer tubes are defined by cylindrical walls and said cavitation fin means are formed as segments of said cylindrical wall.
 5. The invention as defined in claim 4 wherein said cavitation fin means are oriented such that an imaginary radial line passing through the apex thereof is always substantially tangent to an imaginary circle that defines the direction of rotation of the corresponding heat transfer tube.
 6. The invention as defined in claim 1 wherein said means for withdrawing the vapor formed in said evaporation module includes an opening formed in said distribution shaft downstream from said wall so as to permit the vapor formed in said evaporator module to pass therethrough into said distribution shaft downstream of said wall and an effluent conduit means in communication with said distribution shaft for receipt of the vapor therefrom.
 7. The invention as defined in claim 6 wherein said means for condensing the vapor withdrawn from said evaporator module includes a heat exchanger/condenser means.
 8. The invention as defined in claim 1 wherein said distribution shaft means extends through the horizontal axis of said housing.
 9. The invention as defined in claim 8 wherein said rotation means is provided for rotation of said distribution shaft means and said housing about a substantially horizontal axis.
 10. The invention as defined in claim 8 wherein said openings formed in said distribution shaft are sized and spaced so as to cause uniform distribution along the length of said distribution shaft means.
 11. The invention as defined in claim 1 wherein a plurality of the evaporator modules are supported within said solar pond.
 12. The invention as defined in claim 1 wherein said evaporator module is supported from a bridge structure extending transversely across said solar pond.
 13. The invention as defined in claim 12 wherein said evaporator module is secured to said bridge structure by support means for raising said evaporator module to the surface of said solar pond.
 14. A method of purifying raw feedwater, comprising:(a) directing raw feedwater into an evaporator module submerged in a body of heated liquid, said evaporator module having a sealed housing through which a plurality of spaced apart substantially horizontal open ended heat transfer tubes extend; (b) directing heating liquid from said body of heating liquid through said heat transfer tubes; (c) rotating said evaporator module about a substantially horizontal axis; (d) distributing raw feedwater within said evaporator module so as to cause the feedwater to descend into heat transferring contact with said heat transfer tubes and thereby vaporize a portion of the feedwater; (e) withdrawing vapor formed in the evaporator module; and (f) condensing the vapor withdrawn from the evaporator module. 